The thermodynamic optimisation of the Cu–S–Sn system was carried out using the CALPHAD technique. For the liquid phase, an associated solution model, (Cu, Cu 2 S, S, SnS, Sn), was used by assuming the existence of two associates: Cu 2 S and SnS. Ternary compounds Cu 5 Sn 2 S 7 and Cu 10 Sn 2 S 13 were treated as stoichiometric compounds. Ternary compounds Cu 4 SnS 4 , Cu 2 SnS 3 , and Cu 4 Sn 3 S 8 were located on the Cu 2 S–SnS 2 joint and described using the model (Cu 2 S) p (SnS 2 ) q . The Cu 2 Sn 3 S 7 compound was also located on the Cu 2 S–SnS 2 joint and was described as (Cu 2 S, SnS 2 ) considering the range of its homogeneity. With the present thermodynamic parameters, the projection of the liquidus surfaces and invariant reactions of the Cu–S–Sn system were predicted. A set of self-consistent thermodynamic parameters of the Cu–S–Sn system could be obtained.